Red phosphorus is known as a flame retardant for synthetic resins. Red phosphorus flame retardants produced by the surface modification treatment of powdered red phosphorus with various kinds of inorganic or organic compounds and nonflammable resinous compositions containing these flame retardants are known in a large number. Disclosed in U.S. Pat. Nos. 4,242,240, 4,315,897, 4,421,728, and 4,440,880 respectively are a red phosphorus flame retardant treated with a phenol-formaldehyde polycondensate, one treated with aluminum hydroxide and an epoxy resin, one treated with titanium dioxide or titanium phosphate, and one treated with a melamine condensate. The surface modification treatment is conducted in order to improve the chemical stability of red phosphorus and improve the usefulness thereof as a flame retardant.
As is well known, red phosphorus is an unstable substance and undergoes a disproportionation reaction with moisture and oxygen to decompose and thus generate phosphine, which is harmful to the human body. It is further known that the decomposition products other than phosphine adversely affect properties of the flameproofed resin and corrode the mold for use in molding the resin.
The surface modification treatment serves to reduce the generation of such harmful substances by coating the surfaces of the red phosphorus particles with a chemically stable inorganic or organic compound to inhibit the disproportionation reaction.
One of the reasons for the instability of powdered red phosphorus is the shape of the red phosphorus particles which is attributable to the process for the production thereof. The powdered red phosphorus on the market is obtained by heating white phosphorus (the terms "white phosphorus" and "yellow phosphorus" are used interchangeably) in a closed vessel for a long time to convert it into red phosphorus and wet-grinding the resulting solid mass of red phosphorus after completion of the conversion reaction, as described in Kirk-Othmer, "Encyclopedia of Chemical Technology" 2nd Ed., John Wiley & Sons, Inc., New York, 1968, Vol. 15, p.286. Since the pulverizing step is indispensable and the contour of each particle of the powdered red phosphorus is constituted by split-off surfaces having many active sites, the particles are so reactive that the disproportionation reaction and the generation of harmful substances are more accelerated. The red phosphorus flame retardants according to the prior art techniques described above are ones obtained by the surface modification treatment of such red phosphorus powdered by pulverizing.
With recent progress in processes for producing synthetic resin articles and in the character thereof, there is a desire for an inorganic compound flame retardant which is finer and has a narrow distribution of particle size. Such flame retardants are being developed with respect to aluminum hydroxide, boric acid salts, magnesium compounds, molybdenum compounds, antimony trioxide, etc. (Plast. World, 1992, Cahners Publ., Boston Mass., 50(3), p.42.)
The same desire is required of red phosphorus flame retardants, but it has not been realized. This is because the conventional red phosphorus flame retardants produced by surface treatment of finely pulverized red phosphorus are insufficient in stability and unable to meet the requirements of the market, by reason that pulverizing reduces the chemical stability of red phosphorus as described above.
In U.S. Pat. No. 4,879,067 is disclosed a red phosphorus flame retardant based on stable spherical red phosphorus particles having no split-off surfaces. The spherical red phosphorus particles are produced by a process not including a pulverizing step, and there is a description to the effects that the contour of each red phosphorus particle is almost free from a split-off surface and that a more stable flame retardant is obtained from this red phosphorus than that obtained from pulverized red phosphorus.